The present invention relates to fine, electrically conductive lines, grids or circuits embedded in a substrate and a method for making such devices. In one embodiment the conductive grid is sufficiently fine to be invisible to the unaided eye and the substrate is a transparent thermoplastic sheet. Transparent conductive sheets have a variety of uses including resistively heated windows, electromagnetic interference (EMI) shielding, static shielding, antennas, touch screens for computer displays, and surface electrodes for electrochromic windows and liquid crystal displays.
The use of essentially transparent electrically conductive grids for such applications as EMI shielding is well known. The grid can be formed from a network or screen of metal wires that are sandwiched or laminated between transparent sheets or embedded in substrates (U.S. Pat. Nos. 3,952,152; 4,179,797; 4,321,296; 4,381,421; 4,412,255). The disadvantage of using wire screens is the difficulty in handling very fine wires or in making and handling very fine wire screens. For example, a 20 micron diameter copper wire has a tensile strength of only 1 oz (28 grams force) and is therefore easily damaged. Wire screens fabricated with wires of 20 micron diameter are available but are very expensive due to the difficulty in handling very fine wire.
Rather than embed a preexisting wire screen into a substrate, a conductive pattern can be fabricated in situ by first forming a pattern of grooves or channels in a substrate and then filling the channels with a conductive material. This method has been used for making conductive circuit lines and patterns by a variety of means, although usually for lines and patterns on a relatively coarse scale. The grooves can be formed in the substrate by molding, embossing, or by lithographic techniques and then filling the grooves with conductive inks or epoxies (U.S. Pat. No. 5,462,624), with evaporated, sputtered, or plated metal (U.S. Pat. Nos. 3,891,514; 4,510,347; 5,595,943), with molten metal (U.S. Pat. No. 4,748,130), or with metal powder (U.S. Pat. Nos. 2,963,748; 3,075,280; 3,800,020; 4,614,837; 5,061,438; 5,094,811).
These prior art methods have significant limitations, however. For example, one problem with conductive inks or epoxies is that the electrical conductivity is dependent on the formation of contacts between adjacent conductive particles, and the overall conductivity is usually much less than that of solid metal. Vapor deposition of metal or electroplating is generally fairly slow and often requires a subsequent step to remove excess metal that is deposited between the grooves. Molten metal can be placed in the grooves but usually first requires the deposition of some material in the grooves that the metal will wet. Otherwise the molten metal will not penetrate nor stay in the grooves due to surface tension of the molten metal.
Circuits have been made by placing metal powder into grooves followed by compacting the powder to enhance electrical contact between the particles. Lillie et al. (U.S. Pat. No. 5,061,438) and Kane et al. (U.S. Pat. No. 5,094,811) have used this method to form printed circuit boards. However, the method as described by these inventors is not practical for making very fine circuits and metal patterns. The described method forms a pattern of channels in a substrate by embossing the substrate with a patterned tool, places metal powder in the channels, and then uses the same tool to compact the powder. On a fine scale, replacing or re-registering the tool over the embossed pattern to perform the metal compaction would be extremely difficult. For example, a sheet with a pattern of 20 micron wide channels would require that the tool be placed over the pattern to an accuracy of roughly 3 microns from one side of the sheet to the other. For many applications, the sheet may be on the order of 30 cm by 30 cm. Dimensional changes due to thermal contraction of a thermoplastic sheet are typically about 1% or more during cooling from the forming temperature to room temperature. Thus, for a 30 cm by 30 cm sheet, a contraction of 1% would give an overall shrinkage of 0.3 cm. This value is 1000 times larger than the 3 micron placement accuracy needed, making accurate repositioning of the tool impossible.
Alternatively, Lillie et al. (WO85/01231) have suggested placing a deformable material such as plastic over the powder and then applying pressure to the deformable material to compact the metal powder into the grooves. Jack et al. (U.S. Pat. No. 3,075,280) apply pressure to an elastomer sheet over a pattern of particle filled grooves. In these cases, a relatively high pressure must be exerted to the back side of the plastic or elastomeric layer to create sufficient hydrostatic pressure in the region of a groove to cause the compliant layer to press into the groove to compact the powder. Grooves that are deep relative to their width would particularly pose a problem. If the compliant layer is a soft plastic it will engulf the particles making it impossible to remove the compliant layer without removing at least some of the particles.
The present invention is a method for making a fine electrically conductive grid embedded in a polymer substrate. The method includes the steps of providing a polymer substrate, forming a pattern of grooves in the substrate, filling the grooves with electrically conductive powder, and then applying heat and/or pressure to the substrate. The application of heat and/or pressure to the substrate causes the grooves to collapse inward against the conductive powder. Collapsing the grooves serves two purposes. First, it compacts the conductive powder within the groove, thereby establishing a continuously conductive grid line or circuit. Second, as the grooves collapse they become narrower. In applications where transparency is desirable, the narrower grid lines that result allow more light to transmit through the substrate. This inventive method allows grid lines to be made with higher aspect ratios (ratio of line depth to line width) than is possible by previous metal powder methods. The inventive method described herein also allows the fabrication of electrically conductive grids in the surface of materials, such as fluoropolymers, on which it is difficult to adhere metal.
In one embodiment of the invention, sufficient heat and pressure is applied to allow the polymer to completely engulf the compacted powder within the grooves. In this embodiment, the embedded powder becomes isolated from and protected from the environment. Alternatively, the polymer can be allowed to only partially engulf the conductive particles. In certain applications, exposing at least some of the particles at the substrate surface is desirable to allow electrical contact to the conductive grid material.
In another embodiment, the particles can be a metal or alloy that melts at a temperature lower than the temperature applied during the heating and/or pressure step. Normally, fine metal particles that are melted in absence of pressure will not easily coalesce together due to the presence of an oxide skin around the particles. Application of pressure causes the particles to deform sufficiently to break the oxide skin, thereby allowing the molten metal to coalesce and flow along the channels.
For a thermoplastic substrate, the grooves are induced to collapse by means of applying heat and pressure to the flat surface of the substrate. The compressive stress that is created within the body of the thermoplastic substrate distributes itself isostatically within the polymer and causes the grooves to collapse inward from the sidewalls and bottoms of the grooves. The collapsing sidewalls exert pressure on the particles, enhancing their electrical contact or, in the case in which the particles melt, breaking the oxide skin around the now molten particles allowing them to coalesce into a continuous electrical trace.
In a thermoset polymer the molecules are crosslinked, preventing plastic flow. However, conductive traces can still be embedded in the surface of a thermoset polymer substrate by making grooves in the polymer by means of a sharp tool that cuts into the surface. The regions of the polymer adjacent to the grooves retain residual stresses. When the polymer is heated these residual stresses cause the polymer groove to close up to near the original shape of the undisturbed polymer, entrapping any material that was placed into the groove and applying pressure to the material in the grooves. Although in many cases heat alone will collapse the grooves, the collapse can be enhanced by the application of pressure from a flat plate against the substrate. Most of the applied pressure in this case will be against the plowed up regions at the sides or edges of the grooves. In any case, the result is a conductive trace that is narrower than the original width of the groove and up to the same depth as the groove prior to its collapse.